1. Field of the Invention
The present invention relates to a cerium/zirconium-base composite oxide, method for producing the same, and oxygen storage/release component, exhaust gas purification catalyst and exhaust gas purification method using the same, more specifically a cerium/zirconium-base composite oxide (hereinafter sometimes referred to simply as “composite oxide”) useful for, e.g., purification exhaust gases discharged from combustion devices, e.g., internal combustion engines and boilers, and capable of releasing a large amount of oxygen in a low temperature range, method for producing the same, and oxygen storage/release component, exhaust gas purification catalyst and exhaust gas purification method using the same.
2. Description of the Prior Art
Exhaust gases discharged from combustion devices, e.g., internal combustion engines for vehicles and boilers, contain hazardous substances, e.g., hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx), and exhaust gas purification techniques which can simultaneously purify these hazardous substances have been extensively studied. The catalyst used for these techniques is referred to as a three-way catalyst, and some have been already commercialized.
In particular, the three-way catalyst for vehicles is required to efficiently purify these hazardous substances of CO, HC and NOx while flexibly coping with rapidly changing concentrations of these substances, ranging from several ppm to several %.
Oxygen present in exhaust gases plays important roles in systems for catalytically purification these hazardous substances, e.g., promotion of CO and HC oxidation and NOx reduction. However it is difficult to keep constant engine combustion conditions, because of changing driving conditions resulting from changing traffic conditions, with the result that oxygen concentration changes every second. Therefore, a purification system which depends on oxygen will have a deteriorated efficiency for removing these hazardous substances, when its concentration drops. Therefore, the catalyst is incorporated with an oxygen storage/release component (hereinafter referred to as OSC), which releases oxygen into exhaust gases containing oxygen at an insufficient concentration, to improve hazardous substance purification capacity.
CeO2 powder has been used for an OSC, because of its high oxygen storage/release capacity, and demonstrating improved exhaust gas purification efficiency. Extensive studies have been made on improvement of oxygen storage/release capacity and releasing characteristics of CeO2-base powder, e.g., CeO2/ZrO2-base powder, and also on exhaust gas purification catalysts which are incorporated with CeO2-base powder as a promoter.
It is discussed that a cerium/zirconium-base composite oxide of specific atomic composition is effective as an oxygen storage/release component for adjusting exhaust gas purification catalyst functions of oxidation of HC and CO and reduction of NOx (JP-B-6-75675). The document discusses that the composite oxide exhibits high durability to high temperature for exhaust gas oxidation catalysts, when it has a zirconium/cerium (Zr/Ce) atomic ratio in a range from 5/95 to 70/30.
However, conventional knowledge on OSCs is difficult to sufficiently cope with market requirements of improved exhaust gas purification capacity, because needs for catalysts of improved characteristics are ever increasing to satisfy governmental regulations, which are becoming more stringent to cope with environmental problems related to exhaust gases, which are becoming more serious.
An exhaust gas purification catalyst mounted in a vehicle is positioned under a floor, at which temperature is relatively low or directly below the engine where it is exposed to high temperature of exhaust gases immediately after being discharged from the engine. Exhaust gases, which are at high temperature immediately after being discharged from an engine, lose temperature when they reach the underfloor area. These conditions are not favorable for exhaust gas purification, because many exhaust gas purification catalysts generally exhibit a high activity at above a certain high temperature. Nevertheless, however, a catalyst positioned under a vehicle floor is required to exhibit purification capacity even under decreased temperature conditions.
On the other hand, the one positioned directly under an engine may have the exhaust gas purification catalyst components sintered when exposed to exhaust gases, which may be at above 1000° C. Therefore, the catalyst is required to have a stable exhaust gas purification capacity even under severe high-temperature conditions by suppressing sintering of the components. Under these situations, development of OSCs exhibiting excellent performance even under high temperature has been promoted. For example, JP-A-2001-89143 proposes a cerium/zirconium-base composite oxide that oxygen release initiation temperature is at 400° C. or below and that oxygen release amount is not less than 500 μmol/g as an oxygen molecule, even after being exposed to oxidation/reduction cycles at 1000° C.
JP-A 2004-529837 proposes a cerium/zirconium composite oxide having multi-phase cubic crystal habits. It is claimed to have a sufficient high oxygen storage/release capacity for vehicle catalysts, above 300 μmol/g after being exposed to 1000° C. for 4 hours, and high resistance to high temperature.
However, the markets are demanding OSCs capable of releasing oxygen in a low temperature range and having a higher oxygen storage/release capacity, and also demanding exhaust gas purification catalysts of longer service life and higher resistance to high temperature, as exhaust gas regulations are becoming more stringent.
Various types of catalysts have been applied to vehicle exhaust gas purification, to cope with required purification functions. One type of these catalysts simultaneously performs oxidation of CO and HC and reduction of NOx for purification exhaust gases. Such a catalyst is known as a three-way catalyst (hereinafter referred to as TWC). The TWCs generally have a structure with a noble metal, e.g., platinum, rhodium or palladium, loaded on a porous carrier of alumina as a base, which is further loaded on a structured carrier of cordierite or the like.
It is known that a TWC is incorporated with an OSC of cerium/zirconium composite oxide (JP-B-6-75675). However, TWCs of sufficient performance have not been commercialized.